The present invention relates to an optical switch in which an optical signal propagating through a certain optical waveguide is guided to this optical waveguide and another optical waveguide or any desired one selected from a plurality of optical waveguides. More particularly, it relates to an optical switch according to which individual optical signals within optical fibers for propagating light in an optical communication system can be respectively guided to any desired ones of a plurality of other optical fibers.
As prior-art optical switches, there have been proposed one which utilizes optical deflection based on the acoustooptic effect of an optical transmission medium, one which utilizes optical deflection based on the electrooptic effect of a medium, one which changes the coupling coefficient of a directional coupler by virtue of an electrooptic effect, one which comprises a directional coupler and an optical phase modulator in combination, etc. Any of them, however, has not completely satisfied all the fundamental characteristics of an optical waveguide switch such as low loss, low crosstalk and high speed. Moreover, the directional coupler type has had such disadvantages that a high-precision machining technique of 1-2 .mu.m is required, that the length of the optical switch is great and that a long distance is necessitated in order to separate the coupling optical waveguides. The optical deflection type has had such disadvantages that electrodes of complicated structure are required and that a wide separation angle is not attained. These have led to the essential drawback that, when the number of matrices of the optical switch increases, the length of the element exceeds several cm, to incur the increase of transmission loss.
Here, the problems of a deflection type optical switch which is especially closely pertinent to the present invention will be described more in detail with reference to the drawings. The deflection type optical switch is also called a total reflection type optical switch, and the switch of this type is described in, e.g., the official gazette of Japanese Patent Laid-Open No. 54-7951.
FIG. 1 is a plan view showing the deflection type optical switch, while FIG.2 is a sectional view showing a part of section II--II in FIG. 1. In the figures, numeral 1 designates a crystalline substrate of lithium niobate(LiNbO.sub.3) or the like, which has an electrooptic effect, and numerals 2-1, 2-2, 3-1 and 3-2 optical waveguides which are formed in the surface of the crystalline substrate 1 by diffusing a metal such as Ti from the surface of the substrate crystal 1. Numerals 4-1, 4-2, 4-3 and 4-4 indicate the intersection portions of the respectively two optical waveguides intersecting each other. Each of numerals 5-1, 5-2, 5-3 and 5-4 indicates a pair of electrodes which are formed on the surface of the corresponding intersection portion in positions holding the longer diagonal line of the intersection portion therebetween. Although no illustration is made, each electrode is connected to an input terminal by a lead line, whereby a voltage of predetermined value can be applied selectively across the pair of electrodes. Thus, the electrodes construct field applying electrodes which apply an electric field to the corresponding intersection portion of the optical waveguides.
In the optical switch array arranged in this manner, when by way of example light 6 propagating through the optical waveguide 2-1 in the direction of arrow A in FIG. 1 is to be switched by the intersection portion 4-1 into the direction of arrow B so as to propagate through the optical waveguide 3-1, a voltage is applied across the field applying electrodes 5-1, to lower a refractive index in that part of the intersection portion 4-1 of the optical waveguides which is held between the electrodes 5-1. Then, the light 6 is reflected by the part of the lowered refractive index, and the propagating direction thereof is switched into the direction of the arrow B.
In this case, the polarity of the pair of electrodes 5-1 constituting the field applying electrodes 5-1 and the voltage to be applied thereacross differ depending upon the sort of the crystalline substrate 1, the direction of a crystallographic axis, etc., which therefore need to be selected properly. Regarding the extent to which the refractive index is lowered, an angle .theta. defined between the field applying electrode 5-1 and the optical waveguide 2-1 needs to be set so that the incident light may be totally reflected.
In a case where LiNbO.sub.3 is used as the substrate crystal 1 and where the optical waveguides are formed by diffusing Ti sufficiently, the refractive index of the optical waveguide as well as the intersection portion for light having a wavelength of 6328 .ANG. becomes about 2.22. The electrooptical coefficient .gamma. of the substrate crystal 1 is on the order of 30.times.10.sup.-12 m/V, and the refractive index change .DELTA.n of the intersection portion 4-1 based on the electrooptic effect is expressed by: EQU .DELTA.n=1/2n.sup.3.gamma. E
where E denotes an electric field intensity. In this case, therefore, .DELTA.n=0.0005 holds at an electric field intensity of E=5 V/.mu.m. The refractive index of the intersection portion 4-1 to which the electric field is applied becomes about 2.2195, and the total reflection angle is 88.784 degrees. Therefore, the optical switch may be so constructed that the angle .theta. defined between the optical waveguide 2-1 and the field applying electrodes 5-1 becomes 1.216 degree or less. More specifically, when the angle is assumed 1.1 degree, the separation angle (2.theta.) becomes 2.2 degrees, and the optical waveguides 2-1 and 3-1 may be caused to intersect at 2.2 degrees or less. When the electric field intensity to be applied is raised, also the angle .theta. widens. However, even when the high field intensity such as E=50 V/.mu.m is applied, the separation angle (2.theta.) is as small as 7.7 degrees.
Usually, an optical fiber has a diameter of 125 .mu.m or so. In order to couple the optical fiber and the optical switch, the interval between the optical waveguides 2-1 and 2-2 and the interval between the optical waveguides 3-1 and 3-2 taken at the middle positions of the optical waveguides must be at least 125 .mu.m. When the separation angle (2.theta.) is 2.2 degrees, the length (L) of an optical switch portion becomes as great as 3.3 mm, so that optical integration becomes difficult with increase in the number of the optical switch portions. In this manner, the total reflection type has the disadvantage of the very great length of the entire optical switch, besides the high operating voltage of the optical switch. This is attributed to the fact that the electrooptical coefficient of LiNbO.sub.3 is small.
As an expedient for solving the disadvantage, it is considered to employ the single crystal of a material of great electrooptical effect, for example, SBN (Sr-Ba-Nb-O). The crystal, however, might cause a scattering center in the optical waveguide due to an optical damage ascribable to the synergistic action of an electric field and light and is very unstable.
Letter d in FIG. 2 indicates the width of the part in which the refractive index lowers to totally reflect the propagating light when the predetermined voltage is applied across the electrodes (hereinbelow, termed "total reflection part").
The following references are cited to show the state of the art; i) the official gazette of Japanese Patent Laid-Open No. 54-7951,ii) the official gazette of Japanese Patent Laid-Open No. 54-33748,iii) the official gazette of Japanese Patent Laid-Open No. 56-66818.